The invention relates to molded containers made of paper pulp. Such containers must have tapered sidewalls in order to allow their withdrawing from the molding die, and to be stackable to reduce the storage volume. On the other hand automated packing machines used for the containers necessitate an unstacking which is as easy as possible, when done either manually or with a mechanical unstacker.
The containers in the form of boxes, mainly egg cartons, comprise a section with pockets and a cover section which are joined along a common edge acting as a hinge. When stacked the cover sections nest mutually, as do the pocketed sections. Nevertheless, as is well known, nesting is limited to the maximal thickness in the nesting direction, that is to the length of the cut along a plane parallel to said direction, if the element is perfectly rigid. In the case of a slanted wall, the nesting distance is equal to the wall thickness divided by the sine of the slanting angle. With walls of very low thickness approximating 1 millimeter, as those made of molded paper pulp which have been pressure finish-formed after molding, and which have a taper angle over 10.degree., this nesting distance approximates 2.5 millimeters. Moreover, in case of tight nesting, adherence by suction occurs between the smooth surfaces which opposes easy unstacking. To increase the nesting distance i.e. the distance between two stacked containers, it is necessary to give additional thickness to some parts along the stacking direction, additional thicknesses being built up during molding in the pocketed section through pulp settling in the dihedrons between the pockets, settling which generates vertical thicknesses approximating one centimeter.
Accordingly during stacking the cover sections tend to nest mutually to a much higher degree than the pocketed sections. Due to the fact that their bottom surface must be flat, to be able to be printed or labelled, embossings obtained through pulp settlings cannot be used on this part for aesthetic and practical reasons.
Moreover, in view of the hinge between the cover and the pocketed sections, the nesting of the cover sections is not restricted by the nesting of the pocketed sections, and a roof-like inverted folding occurs which is accentuated when the number of stacked cartons is higher, and unstacking proves to be more difficult and hazardous.